Maximising spectrum utilisation has been a prime goal in the design of modern mobile satellite systems in recent years. The spectrum allocated to Mobile Satellite Service (MSS) is a limited resource and needs to be shared among competing satellite system operators. Therefore maximising spectrum utilisation and frequency reuse have become a major consideration in the design of modern MSS constellations. Another major consideration is dimensioning of user terminals, with a drive for progressively smaller and more compact terminals.
In order to accommodate both considerations the MSS satellite designs have become progressively more complex, utilising a large number of spot beams, effectively overlaying the satellite coverage area with a large array of cells on the ground. Smaller spot beams increase the satellite power concentrated in a cell and therefore allow for smaller user terminals. A larger number of spot beams and corresponding cells in the coverage area allows for more effective spectrum utilisation and for higher frequency reuse factors to be achieved, since a carrier frequency may be re-allocated in nearby beams, provided that adequate isolation is achieved between those beams, thereby limiting inter-cell interference to an acceptable level.
Multi-User Detection (MUD) is another technological advancement that could contribute to maximising spectrum utilisation. Originally MUD techniques were developed for Code Division Multiple Access (CDMA), utilising the discrimination in the signal code space for detecting individual user signals in a simultaneous reception of multi-user signals. Recent developments in MUD have found solutions for discriminating and detecting individual signals in a simultaneous reception of multi-user signals where those signals are not CDMA coded; see for example CA-A-2201460 which discloses a technique which relies on forward error correction (FEC). The paper ‘Multiuser Decoding for Multibeam systems’ by M. Moher, IEEE Transactions on Vehicular Technology, July 2000, Volume 49, Number 4 discloses a technique which exploits other diversity dimensions, such as signal reception diversity through more than one transmission path; the same spectrum is allocated to all beams in a multibeam satellite or terrestrial cellular system.
Recent satellite systems incorporate position location capabilities in the user terminal, utilising Global Positioning System (GPS) receivers or alternative global navigation satellite system (GNSS) receivers. A Time Division Multiple Access (TDMA) system may use the knowledge of user terminal position in optimising system performance and economising resources allocated to the user. One area of economising is in allowing new users to log on to the system in a narrow time window by making use of the precise position and time determination capabilities of GNSS systems such as GPS.
The radio resource management (RRM) in a packet data system may include allocation of shared bearers to cells (which are served by spot beams in a satellite system) in response to aggregate capacity requests from users in the cells. Bearer allocation is based on a frequency plan that aims to maximise frequency reuse in the spectrum constraints of the system. In reusing a bearer frequency between cells the frequency plan takes into account the required isolation between cells to maintain interference; at an acceptable level.
In a terrestrial cellular system, the frequency plan may be implemented at a mobile switching centre, which is connected to base stations having RF transmitters and receivers which define the cell pattern of the system. The mobile switching centre stores frequency plan data and allocates channels to user terminals in accordance with the frequency plan and demand for channels by the user terminals.
In a multibeam satellite system, the frequency plan may be implemented on a satellite, or by one or more ground stations if the satellites are ‘dumb’ or repeater satellites. For example, in the Inmarsat™ systems channel allocation may be performed by a Land Earth Station (LES) or Network Coordination Station (NCS).
Two examples of frequency reuse patterns for bearer frequencies are shown in FIG. 1 and FIG. 2.
FIG. 1 shows a frequency reuse pattern in a 5-cell array. Bearer frequencies are reused between cells 1 and 5 and between cells 2 and 4, taking advantage of the isolation between those cells.
FIG. 2 shows a frequency reuse pattern in a large cellular array, in which one hexagonal 7-cell cluster is highlighted. The cells within each cluster are numbered from 1 to 7. Given that the required separation distance S between cell centres (for achieving required isolation and limiting interference) is √7D, where D is the distance between adjacent cell centres, a 7-cell reuse pattern can be applied, allowing for a bearer frequency to be used once in any hexagonal 7-cell cluster, and resulting with a reuse factor of 1/7. Alternatively, clusters of 3, 4, 6, 19 or other numbers of cells may be used, depending on the required separation distance D.
The radio resource management (RRIM) in a packet data system may also include allocation of return link shared bearer time slots to users in corresponding cells. Users in a given cell are allocated time slots in the shared bearer serving that cell, for example by transmitting an allocation message to a user terminal defining the frequency and time slot to be used by that user terminal. Initial access channels, which are typically used by terminals to establish contact with the system and to transmit a channel allocation request, may have a fixed frequency and time slot known a priori by the mobile terminal or a variable frequency and time slot indicated on a fixed channel, such as a bulletin board channel.
Examples of time slot allocation methods are:                a. allocation on an exclusive basis (i.e. a given time slot is dedicated to one user);        b. allocation on contention basis (i.e. more than one user may use a given time slot). A contention channel is normally not loaded more than 10% in order to limit probability of collisions to an acceptable level;        c. use of an initial access channel, where a relatively long time slot is required due to time uncertainty at the user terminal. Access through this channel is also subject to probability of collisions between competing packets trying to use the same time slot for initial access.        
FIG. 3 illustrates the time slot allocation function in the case of adjacent cells c1 and c2 with corresponding shared bearers f1 and f2. Users u1, u2 in c1 are allocated exclusive time slots f1-T1, f1-T2. Users u3, u4 are allocated contention time slot f1-Tc. Users u5, u6 are attempting initial access using time slot f1-TA. Similarly users in cell c2 are using exclusive, contention and initial access time slots in shared bearer f2.
The shared bearer frequencies allocated to the cells may be reused in the cell array in keeping with the isolation requirements for limiting inter-cell interference.
The document WO 91/01073 describes a cellular radio system in which each cell may be subdivided into a pair of sectors with carriers being shared between the pair of sectors.